Granite - Igneous Rock Types Igneous Rock Types Pictures of Igneous Rock Types Use Deep Earth and Other Planets Granite consists of quartz (gray), plagioclase feldspar (white) and alkali feldspar (beige) plus dark minerals, in this case biotite and hornblende . (more below) "Granite" is used by the public as a catch-all name for any light-colored, coarse-grained igneous rock. The geologist examines these in the field and calls them granitoids pending laboratory tests. The key to true granite is that it contains sizable amounts of quartz and both kinds of feldspar. This article goes much deeper into granite . This granite specimen comes from the Salinian block of central California, a chunk of ancient crust carried up from southern California along the San Andreas fault. Pictures of other granite specimens appear in the granite picture gallery . Also see the granite landforms of Joshua Tree National Park . And big closeup pictures of granite are available in the closeup rock wallpaper photos .

Photo Credit: Photo (c) 2004 Andrew Alden, licensed to About.com ( fair use policy ) Granite: Igneous Rock - Pictures, Definition & More Granite What is Granite? What is Granite Used For? Granite: The specimen above is a typical granite. It is about two inches across. The grain size is coarse enough to allow recognition of the major minerals. The pink grains are orthoclase feldspar, and the clear to smoky grains are quartz or muscovite. The black grains can be biotite or hornblende. Numerous other minerals can be present in granite. What is Granite? Granite is a light-colored igneous rock with grains large enough to be visible with the unaided eye. It forms from the slow crystallization of magma below Earth's surface. Granite is composed mainly of quartz and feldspar with minor amounts of mica, amphiboles, and other minerals. This mineral composition usually gives granite a red, pink, gray, or white color with dark mineral grains visible throughout the rock. Granite in Yosemite Valley: Photograph of Yosemite Valley, California, showing the steep granite cliffs that form the walls of the valley. © iStockphoto / photo75.

The Best-Known Igneous Rock Granite is the best-known igneous rock. Many people recognize granite because it is the most common igneous rock found at Earth's surface and because granite is used to make many objects that we encounter in daily life. These include counter tops, floor tiles, paving stone, curbing, stair treads, building veneer, and cemetery monuments. Granite is used all around us - especially if you live in a city. Related: Explore the many uses of granite! Granite is also well-known from its many world-famous natural exposures. These include: Stone Mountain, Georgia; Yosemite Valley , California; Mount Rushmore, South Dakota; Pike's Peak, Colorado; and White Mountains, New Hampshire. Rock & Mineral Kits: Get a rock, mineral, or fossil kit to learn more about Earth materials. The best way to learn about rocks is to have specimens available for testing and examination. Yosemite Nature Notes - Granite: This video examines some of the granites that create the scenery and climbing pleasures of Yosemite National Park. Yosemite Nature Notes - Granite: This video examines some of the granites that create the scenery and climbing pleasures of Yosemite National Park. Granite: Photograph of a white, fine-grained granite.

This specimen is about two inches across. Multiple Definitions of Granite The word "granite" is used in a variety of ways by different people. A simple definition is used in introductory courses; a more precise definition is used by petrologists (geologists who specialize in the study of rocks); and, the definition of granite expands wildly when used by people who sell decorative stone such as countertops, tile, and building veneer. These multiple definitions of granite can lead to communication problems. However, if you know who is using the word and who they are communicating with, you can interpret the word in its proper context. Three common usages of the word "granite" are explained below. Granite close up: Magnified view of the white, fine-grained granite from the photograph above. The area shown in this image is about 1/4 inch across. A) Introductory Course Definition Granite is a coarse-grained, light-colored igneous rock composed mainly of feldspars and quartz with minor amounts of mica and amphibole minerals. This simple definition enables students to easily identify the rock based upon a visual inspection.

B) Petrologist's Definition Granite is a plutonic rock in which quartz makes up between 10 and 50 percent of the felsic components and alkali feldspar accounts for 65 to 90 percent of the total feldspar content. Applying this definition requires the mineral identification and quantification abilities of a competent geologist. Many rocks identified as "granite" using the introductory course definition will not be called "granite" by the petrologist - they might instead be alkali granites, granodiorites, pegmatites , or aplites. A petrologist might call these "granitoid rocks" rather than granites. There are other definitions of granite based upon mineral composition. The chart below illustrates the range of granite compositions. From the chart you can see that orthoclase feldspar , quartz , plagioclase feldspar , micas, and amphiboles can each have a range of abundances. Pegmatite: Photograph of a granite with very large crystals of orthoclase feldspar. Granites with such large crystals are known as "pegmatites." This rock is about two inches across.

Granite composition chart: This chart illustrates the generalized mineral composition of igneous rocks. Granites and rhyolites (compositionally equivalent to granite but of a fine grain size) are composed mainly of orthoclase feldspar, quartz, plagioclase feldspar, mica, and amphibole. "Granite": All of the rocks above would be called "granite" in the commercial stone industry. Clockwise from top left they are: granite , gneiss , pegmatite , and labradorite . Click on any of their names above for an enlarged view. Each of the images above represents a slab of polished rock about eight inches across. C) Commercial Definition The word "granite" is used by people who sell and purchase cut stone for structural and decorative use. These "granites" are used to make countertops, floor tiles, curbing, building veneer, monuments, and many other products. In the commercial stone industry, a "granite" is a rock with visible grains that is harder than a marble .

Under this definition, gabbro , basalt , pegmatite , schist , gneiss , syenite, monzonite, anorthosite, granodiorite, diabase, diorite , and many other rocks will be called "granite." The collection of images here illustrates the range of rocks that might be called "granite." Granite counter tops: Granite counter tops in a new kitchen. Image © iStockphoto / Bernardo Grijalva. Mount Rushmore: Mount Rushmore in the Black Hills, South Dakota is a sculpture of United States presidents George Washington, Thomas Jefferson, Theodore Roosevelt, and Abraham Lincoln sculpted from a granite outcrop. Image © iStockphoto / Jonathan Larsen. Uses of Granite Granite is the rock most often quarried as a "dimension stone" (a natural rock material that has been cut into blocks or slabs of specific length, width, and thickness). Granite is hard enough to resist most abrasion, strong enough to bear significant weight, inert enough to resist weathering, and it accepts a brilliant polish. These characteristics make it a very desirable and useful dimension stone.

Most of the granite dimension stone produced in the United States comes from high-quality deposits in five states: Massachusetts, Georgia, New Hampshire, South Dakota, and Idaho. Granite has been used for thousands of years in both interior and exterior applications. Rough-cut and polished granite is used in buildings, bridges, paving, monuments, and many other exterior projects. Indoors, polished granite slabs and tiles are used in countertops, tile floors, stair treads, and many other practical and decorative features. Related: Azurite Granite? A Gemstone? High price often reduces the popularity of a construction material, and granite often costs significantly more than man-made materials in most projects. However, granite is frequently selected because it is a prestige material, used in projects to produce impressions of elegance, durability, and lasting quality. Granite is also used as a crushed stone or aggregate. In this form it is used as a base material at construction sites, as an aggregate in road construction, railroad ballast, foundations, and anywhere that a crushed stone is useful as fill. Granitic rocks: This triangular diagram is a classification method for granitic rocks. It is based upon the relative abundance of feldspars (K-Na-Ca) and quartz.

Mafic elements are not considered. It is modified after a classification chart prepared by the International Union of Geological Sciences. Image and modification by the United States Geological Survey. Granite in the Continental Crust Most introductory geology textbooks report that granite is the most abundant rock in the continental crust. At the surface, granite is exposed in the cores of many mountain ranges within large areas known as "batholiths," and in the core areas of continents known as "shields." The large mineral crystals in granite are evidence that it cooled slowly from molten rock material. That slow cooling had to have occurred beneath Earth's surface and required a long period of time to occur. If they are today exposed at the surface, the only way that could happen is if the granite rocks were uplifted and the overlying sedimentary rocks were eroded. In areas where Earth's surface is covered with sedimentary rocks, granites, metamorphosed granites, or closely related rocks are usually present beneath the sedimentary cover. These deep granites are known as "basement rocks." Granite Granite Granite is a common intrusive plutonic igneous rock. Intrusive means that it has moved into other rocks by force coming up from the mantle.

Plutonic means that it is magma that does not reach the surface of the earth and so cools very slowly underground. Because it cools slowly crystals have time to form. Its name comes from the Latin word granum which means “a grain” for the coarse grain crystalline structure of the rock. This rock has made its way into the earth’s crust during all geologic periods. Most of the granite we see today had its origin during Precambrian times. Granite is made up of quartz, mica, and feldspar. It comes in a wide variety of colors including reds, browns, and many shades of gray from almost black to nearly all white. There are many types of granite. The type of granite a particular specimen is depends upon the percentages of minerals that make up the rock, especially quartz, alkali feldspar and plagioclase feldspar. The types are given letter designations that generally indicate what kind of parent rock went into the making of the granite. So we could have an I-type granite that was formed from an igneous parent rock, an S-type granite that was formed from sedimentary parent rock or an M-type that is derived from rocks that are from the mantle.

This rock makes up a large part of the continental plates. It is lighter than the basalts of the oceanic plates and so it floats over them. It is the most common bedrock in continental plates. It is a very hard and durable stone and so it is used in construction as a building stone. It is also used to make floor tiles. Some granites can be polished and make beautiful counter tops and tables. Until the 18 th century granite could only be carved by hand. When steam powered cutting and dressing tool were developed by Alexander MacDonald in Aberdeen, Scotland in 1832 granite became a popular decorative stone. It was used to make head stones in Cemeteries across the United Kingdom. These granite gravestones were considered to be a status symbol by those in Victorian Britain. Now it is carved with computer controlled machines or sandblasted using rubber coatings to protect polished surfaces. Granite is a favorite for rock climbers. Its faces are steep, footings are reliable, and there are extensive crack systems. There are many areas with popular granite faces attracting climbers today like Half Dome in Yosemite National Park pictured here. [ ?

]Subscribe To This Site All About Granite and its Geology By Andrew Alden Updated February 15, 2016. Granite is the signature rock of the continents. More than that, granite is the signature rock of the planet Earth itself. The other rocky planets— Mercury , Venus and Mars—are covered with basalt , as is the ocean floor of Earth. But only Earth has this beautiful and interesting rock type in abundance. Granite Basics Three things distinguish granite. First, granite is made of large mineral grains (its name is Latin for "granum," or "grain") that fit tightly together. It is phaneritic , meaning its individual grains are large enough to distinguish with the human eye. Second, granite always consists of the minerals quartz and feldspar , with or without a wide variety of other minerals (accessory minerals). The quartz and feldspar generally give granite a light color, ranging from pinkish to white. That light background color is punctuated by the darker accessory minerals. Thus, classic granite has a "salt-and-pepper" look. The most common accessory minerals are the black mica biotite and the black amphibole hornblende .

continue reading below our video What are the Seven Wonders of the World Third, almost all granite is igneous (it solidified from magma ) and plutonic (it did so in a large, deeply buried body or pluton). The random arrangement of grains in granite—its lack of fabric—is evidence of its plutonic origin. Other igneous, plutonic rocks, like granodiorite, monzonite, tonalite and quartz diorite, have similar appearances. A rock with a similar composition and appearance as granite, gneiss , can form through long and intense metamorphism of sedimentary (paragneiss) or igneous rocks (orthogneiss). Gneiss, however, is distinguished from granite by its strong fabric and alternating dark and light colored bands. Amateur Granite, Real Granite and Commercial Granite With only a little practice, you can easily tell this kind of rock in the field. A light-colored, coarse-grained rock with a random arrangement of minerals—that's what most amateurs mean by "granite." Ordinary people and even rockhounds agree. Geologists, however, are professional students of rocks, and what you would call granite they call granitoid .

True granite, which has a quartz content between 20 and 60 percent and a greater concentration of alkali feldspar than plagioclase feldspar , is only one of several granitoids. Stone dealers have a third, much-different set of criteria for granite. Granite is a strong stone because its mineral grains have grown tightly together during a very slow cooling period. Additionally, the quartz and feldspar that compose it are harder than steel . This makes granite desirable for buildings and for ornamental purposes, such as gravestones and monuments. Granite takes a good polish and resists weathering and acid rain . Stone dealers, however, use "granite" to refer to any rock with big grains and hard minerals, so many types of commercial granite seen in buildings and showrooms don't match the geologist's definition. Black gabbro , dark-green peridotite or streaky gneiss, which even amateurs would never call "granite" in the field, still qualify as commercial granite in a countertop or building. How Granite Forms Granite is found in large plutons on the continents, in areas where the Earth's crust has been deeply eroded.

This makes sense, because granite must cool very slowly at deeply buried locations to produce such large mineral grains. Plutons smaller than 100 square kilometers in area are called stocks, and larger ones are called batholiths. Lavas erupt all over the Earth, but lava with the same composition as granite ( rhyolite ) only erupts on the continents. That means that granite must form by the melting of continental rocks. That happens for two reasons: adding heat and adding volatiles (water or carbon dioxide or both). Continents are relatively hot because they contain most of the planet's uranium and potassium, which heat up their surroundings through radioactive decay. Anywhere that the crust is thickened tends to get hot inside (for instance in the Tibetan Plateau ). And the processes of plate tectonics , mainly subduction , can cause basaltic magmas to rise underneath the continents. In addition to heat, these magmas release CO2 and water, which helps rocks of all kinds melt at lower temperatures. It is thought that large amounts of basaltic magma can be plastered to the bottom of a continent in a process called underplating.

With the slow release of heat and fluids from that basalt, a large amount of continental crust could turn to granite at the same time. Two of the most well-known examples of large, exposed granitoids are Half Dome and Stone Mountain . What Granite Means Students of granites classify them in three or four categories. I-type (igneous) granites appear to arise from the melting of preexisting igneous rocks , S-type (sedimentary) granites from melted sedimentary rocks (or their metamorphic equivalents in both cases). M-type (mantle) granites are rarer and are thought to have evolved directly from deeper melts in the mantle. A-type (anorogenic) granites now appear to be a special variety of I-type granites. The evidence is intricate and subtle, and the experts have been arguing for a long time, but that is the gist of where things stand now. The immediate cause of granite collecting and rising in huge stocks and batholiths is thought to be the stretching apart, or extension, of a continent during plate tectonics. This explains how such large volumes of granite can enter the upper crust without exploding, shoving or melting their way upward.

And it explains why the activity at the edges of plutons appears to be relatively gentle and why their cooling is so slow. On the grandest scale, granite represents the way the continents maintain themselves. The minerals in granitic rocks break down into clay and sand and are carried to the sea. Plate tectonics returns these materials through seafloor spreading and subduction, sweeping them beneath the edges of the continents. There they are rendered back into feldspar and quartz, ready to rise again to form new granite when and where the conditions are right. It is all part of the never-ending rock cycle . Rock Types Rock Types About Rock types Rock type is usually defined as a particular kind of rock having a specific set of characteristics 1 . Rock types are specific assemblages of minerals (most rocks are composed of minerals). Rocks are much more vaguely defined than minerals . Even rocks within one rock type may have a highly variable composition. Some important igneous rocks from left to right: gabbro, andesite, pegmatite, basalt, pumice, porphyry, obsidian, granite, and tuff.

Still, there is a fundamentally sound reason to talk about such a vague concept as a rock type because these assemblages occur again and again in many different locations. Rock type is like a biome (desert, savannah, rain forest). One particular desert may greatly differ from another (one is sandy and another rocky), but they both share something that is common (low precipitation). Exactly the same is true with rock types — one granite may be white and another red, but they both have a similar composition (major minerals are feldspar and quartz). Here is a list of major rock types recognized by geologists. Immense number of rock types have been defined (their definitions often overlapping) and many rock classifications exist. However, there seems to be a fair number of terms and classification principles that have successfully stood the test of time.

Contents Igneous rocks Amphibolite · Anthracite · Blueschist · Charnockite · Eclogite · Epidosite · Fenite · Gneiss · Granulite · Greenschist · Greisen · Hornfels · Marble · Metapelite · Migmatite · Mylonite · Phyllite · Quartzite · Schist · Serpentinite · Shungite · Skarn · Slate · Soapstone · Suevite Igneous rocks It all starts with igneous rocks . The Earth is believed to have been entirely molten in its early stages of development. Hence, all other rock types are derivatives of igneous rocks. Andesite is a volcanic rock with a so-called average composition. It is more felsic than basalt and more mafic than dacite. The main minerals are plagioclase and hornblende. Andesite is a common rock of subduction zone volcanoes. It is generally dark gray and may contain phenocrysts of plagioclase (usually andesine), biotite, and hornblende. Andesite is an extrusive equivalent of diorite.

^ Anorthosite is a plutonic rock which is composed of plagioclase (>90%). Anorthosite is a cumulate rock. Its constituent plagioclase crystals floated to the top of the mafic magma chamber and crystallized there as almost monomineralic plagioclase rock. Some anorthosites display very beautiful display of colors known as labradorescence and are therefore valuable ornamental stones. Anorthosite is a common rock on the Moon. The highlands of the Moon which we see as brighter areas are largely composed of this rock type. ^ Aplite is a fine-grained and usually light-colored plutonic rock. Aplite usually solidified in a narrow crack where heat loss to the adjacent rocks was rapid and crystals did not have much time to grow in size. Aplites may have a variable composition from granite to gabbro, but the term “aplite” is mostly used to describe fine-grained granitic rocks. ^ Basalt is a very common dark-colored volcanic rock composed of calcic plagioclase (usually labradorite), clinopyroxene (augite) and iron ore (titaniferous magnetite).

Basalt may also contain olivine, quartz, hornblende, nepheline, orthopyroxene, etc. Basalt is a volcanic equivalent of gabbro. Oceanic crust is mostly composed of basalt or rocks with the same composition (gabbro, diabase). ^ Basanite is a volcanic rock. It is similar to basalt but contains less silica and usually more alkali metals. Basanite may contain olivine, pyroxene, and feldspathoid phenocrysts. Similar rock without olivine is tephrite. Limburgite is a glassy nepheline basanite. ^ Carbonatite is an igneous rock that is predominantly composed of carbonate minerals. Both volcanic and plutonic carbonatites are known. Alvikite is a volcanic calcite carbonatite, sövite is a plutonic calcite carbonatite. Carbonatite is generally associated with alkaline igneous rocks. Carbonatite is a host rock of several important commodities, most notably apatite, rare earth elements and niobium. ^ Dacite is a felsic volcanic rock intermediate in composition between andesite and rhyolite.

Dacite is a common rock of subduction-related volcanoes on the active continental margins (Cascades in the northwestern USA, for example). Dacitic magma in the crust solidifies as intrusive rock granodiorite. ^ Diabase is a dark-colored igneous rock. It is compositionally equivalent to gabbro and basalt but texturally between them. Diabase is a common rock type. It occurs mostly in shallow intrusions (dikes and sills) of basaltic composition. Diabase grades to basalt when it solidifies rapidly and to gabbro when more time is given to the crystals to grow. ^ Diorite is a plutonic rock with sodium-rich plagioclase and hornblende. Diorite is compositionally equivalent to volcanic rock andesite. It is intermediate in composition between felsic granite and mafic gabbro. ^ Dunite is an ultramafic plutonic rock that is composed almost exclusively of olivine (>90%). Dunite is a sub-type of peridotite. These rocks are abundant in the Earth’s mantle but occur relatively rarely in continental rocks. Olivine rock is a synonym of dunite.

Olivinite is a variety that contains more magnetite than chromite. ^ Foyaite is a rare igneous rock consisting of nepheline and alkali feldspar with a trachytic texture. Foyaite is a variety of nepheline syenite. ^ Gabbro is a coarse-grained and usually dark-colored igneous rock. Gabbro is an intrusive rock. Igneous rocks with a similar composition are basalt (extrusive equivalent of gabbro) and diabase (the same rock type could be named dolerite or microgabbro instead). ^ Granite is a very common plutonic rock with feldspar and quartz as the main constituents. Granite is an igneous rock, but some high-grade metamorphic rocks are compositionally very close and have been sometimes referred to as granites also. ^ Granodiorite is a granitoid (quartz-rich plutonic rock) in which the dominant feldspar is plagioclase. Granodiorite is compositionally between other granitoids granite and tonalite. Granodiorite contains more quartz than diorite. This rock type is frequently misidentified as granite.

^ Harzburgite is what remains when basalt is taken out of the mantle. Harzburgite is a sub-type of peridotite. Harzburgite is composed of orthopyroxene and olivine, it is one of the ultramafic rocks and belongs into the peridotite group. These are rocks that are abundant in the mantle but scarce on the surface. ^ Hawaiite is a volcanic rock similar to basalt but contains more alkali metals. It is a sodic variety of trachybasalt. ^ Hornblendite is an ultramafic plutonic igneous rock that is composed almost entirely of hornblende. Hornblendite crystallized from water fluid-rich magma which favored the formation of hydrous hornblende over anhydrous pyroxene. ^ Ignimbrite is a fiery rock. It is a pyroclastic rock formed by very hot ground-hugging cloud of volcanic ash, blocks, and gases known as pyroclastic flow or pyroclastic density current. Ignimbrite is synonymous with flood tuff, welded tuff, ash-flow tuff and pyroclastic flow deposit 3 .

^ Ijolite is a rare igneous rock consisting of nepheline and pyroxene. Ijolite is a variety of foidolite (feldspathoid-rich plutonic rock in which fledspathoids are dominating over feldspars). ^ Kimberlite is an ultramafic rock consisting of olivine and one or more of the following: phlogopite, serpentine, diopside, monticellite, calcite. Kimberlite contains no leucite. Kimberlite is an economically important rock type because it is the principal source (host rock) of diamonds. Kimberlite occurs in pipe-like rock body called diatreme, which formed through violent gas-charged volcanic eruption. ^ Komatiite is an ultramafic lava. Komatiite is compositionally similar to intrusive rock peridotite. Some volcanic rocks (picrites) may be ultramafic because they are olivine cumulates, but komatiite is a true ultramafic lava that formed as a result of very extensive partial melting. Komatiites do not form anymore. Most likely because the Earth has cooled down and does not allow such a high degree of partial melting to take place.

Komatiite lavas commonly demonstrate spinifex texture — long olivine crystals in the upper part of a lava flow. ^ Limburgite is a variety of basanite with a glassy groundmass and nepheline as the main feldspathoid mineral. Limburgite contains no feldspars. ^ Monzonite is a plutonic rock intermediate in composition between syenite and diorite. Monzonite contains approximately equal amount of K-feldspar and plagioclase. Monzonite contains no or small amount of quartz and the content of alkali feldspar and plagioclase are roughly equal. ^ Nepheline syenite is a plutonic rock rich in alkali metals and relatively poor in silica. As a result of that it contains alkali minerals like feldspathoid nepheline in addition to alkali feldspar, alkali amphibole and/or alkali pyroxene. Quartz is absent. Phonolite is a volcanic analogue of nepheline syenite. ^ Norite is a gabbroid rock in which the pyroxene mineral is orthopyroxene. ^ Obsidian is a volcanic glass — a rock type that in pure form contains no crystals.

Obsidian is usually rhyolitic in composition and black or reddish in color. Obsidian is frequently associated with pumice. ^ Pegmatite is an exceptionally coarse-grained plutonic rock. Most pegmatites have a granitic composition. Simple pegmatites contain common minerals like feldspar, mica and quartz. Complex pegmatites are rich in unusual chemical elements and minerals which makes pegmatite a potential mineral resource. ^ Peridotite is the principal rock type in the Mantle. Unaltered peridotite is usually dark green in color but most peridotites we see in the surface are altered to a metamorphic rock type serpentinite. There are several sub-types of peridotite: harzburgite, dunite, lherzolite, etc. ^ Phonolite is an alkaline volcanic rock that is composed of fine-grained alkali feldspar, nepheline, alkali amphiboles and alkali pyroxenes. Phonolite may be porphyritic. It is an extrusive equivalent of plutonic rock nepheline syenite. ^ Picrite is a mafic igneous rock similar to basalt but with more olivine.

Picrite may form as an olivine cumulate rock. Olivine crystals may settle to the lower part of a lava flow, forming an ultramafic rock. ^ Porphyry is an igneous rock characterized by porphyritic texture. Porphyritic texture is a very common texture in igneous rocks in which larger crystals (phenocrysts) are embedded in a fine-grained groundmass. Porphyritic texture indicates that the crystallization process started well before the lava was extruded to the surface. ^ Pumice is a lava froth — an extremely vesicular and light-weight volcanic rock with a variable composition. Pumice is unusually light-weight for a rock — it floats in water. Pumice is usually light-colored and has a felsic composition. Pumice forms during explosive volcanic eruptions. ^ Pyroxenite is a plutonic rock that is composed almost exclusively (over 90%) of pyroxene minerals. Pyroxenite is a cumulate rock usually associated with other ultramafic (peridotite) and mafic (gabbro) rocks.

^ Rapakivi is a variety of granite with K-feldspar phenocrysts that are mantled with plagioclase. Rapakivi is an anorogenic rock occurring in intra-plate settings. ^ Rhomb-porphyry is a rare porphyritic rock with abundant wedge-shaped anorthoclase phenocrysts. Rhomb-porphyry from Norway is a volcanic equivalent of larvikite. Rhomb-porphyry is compositionally trachyandesite (latite). ^ Rhyolite is a felsic volcanic rock. It is an extrusive analogue of granite. Rhyolite is associated with explosive volcanoes. Rhyolitic rock, because of high viscosity, may occur as pumice and obsidian. Porphyritic rocks are also frequently rhyolitic in composition. ^ Scoria is a highly vesicular mafic dark-colored volcanic rock. It is similar to pumice but darker in color, not as light-weight, and more mafic in composition. Vesicules in scoria are usually larger than in pumice. ^ Sövite is a type of plutonic carbonatite where the dominant carbonate mineral is calcite.

Sövite is the most common variety of carbonatite. It is often mined as a host rock of various mineral resources. ^ Syenite is a plutonic rock that contains little or no quartz and in which K-feldspar dominates over plagioclase. Syenite is like a granite but without quartz. In nepheline syenite some of the alkali feldspar is replaced by nepheline and quartz is completely absent. ^ Tephrite is a volcanic rock resembling basalt, but it is rich in feldspathoid minerals. This makes it very similar to basanite, but tephrite contains very little olivine. ^ Tonalite is a granitoid in which almost all the feldspar (>90%) is plagioclase. Trondhjemite and plagiogranite are varieties of tonalite. ^ Trachyandesite is a volcanic rock intermediate in composition between trachyte and andesite. Trachyandesite is compositionally close to intrusive rock monzonite. Rhomb-porphyry is a variety of trachyandesite. ^ Trachyte is a feldspar-rich rock.

It is a volcanic equivalent of syenite. Trachyte grades into rhyolite with increasing quartz content. ^ Troctolite is a variety of gabbro without pyroxene. Troctolite is a cumulate rock associated with other gabbroic rocks like gabbro, norite, and anorthosite. ^ Trondhjemite is a leucocratic variety of tonalite composed of sodic plagioclase, quartz and biotite. ^ Tuff is a volcanic analogue of sandstone. It is a consolidated and lithified volcanic ash. ^ Sedimentary rocks Sedimentary rocks are products of the consolidation of loose sediments which are in most part bits and pieces of other disintegrated rocks. There are three main types of sedimentary rocks: mudstone, sandstone, limestone. Sure, this is very simplistic approach, but volumetrically almost all sedimentary rocks fall under these categories if we give them relatively broad definition e.g. sandstone also includes siltstones and conglomerates, and limestone includes all sedimentary carbonate rocks. There are notable exception that do not fit into this classification scheme like evaporites, coal, and chert, but volumetrically they are clearly less important.

In the list below, of course, these exceptions are not neglected. Arkose is a feldspar-rich (25%) sandstone. Arkose is usually coarse-grained and composed of angular grains. These properties are characteristic of relatively immature sediments. Arkose is usually a disintegration product of nearby granitic rocks. ^ Banded iron formation is a chemogenic metasedimentary rock consisting of alternating silica- and iron-rich bands. Banded iron formation (BIF) is the principal source of iron. It is perhaps the most important rock type as our society is heavily reliant on iron extracted from it. ^ Bauxite is an aluminum-rich sedimentary rock. It is a principal ore of aluminum. Aluminum in bauxite is hosted by aluminum hydroxide minerals, mostly gibbsite. The major impurities are iron oxides and hydroxides (which give reddish color to most bauxites) and clay minerals. Bauxite is a weathering product of aluminum-bearing rocks. ^ Bog iron is a type of iron ore composed of limonite with impurities like clay or plant debris. Bog iron is porous rock formed in bogs, wetlands, and shallow lakes.

Iron is precipitated from water by the oxidizing action of bacteria and algae. Bog iron is common in cold temperate zones of Scandinavia and North America. ^ Breccia is a coarse-grained shattered rock that is composed of highly angular fragments held together by a mineral cement. Breccia may be a result of several different geological processes: talus formation, explosive volcanic eruptions, tectonic movements, rock avalanches, etc. ^ Chalk is a soft, friable and porous variety of limestone. Chalk is composed of calcareous tests of marine microorganisms like coccoliths and foraminifers. The best known chalks are of Cretaceous age. Chalk is often associated with chert (flint). ^ Chert is a hard and compact sedimentary rock, consisting dominantly of very small quartz crystals. Chert occurs mostly in carbonate sedimentary rocks as nodules or layers. Flint is a dark-colored variety of chert. ^ Coal is a combustible rock containing more than 50% by weight of carbonaceous material. Coal in most cases is a lithified peat. Coal is a major fossil fuel, although in recent years its importance in the developed world has been declining.

^ Conglomerate is a lithified gravel (rounded rock fragments or mineral grains larger than 2 mm in size). Conglomerate contains smaller grains (matrix) in the interstices. Conglomerate is a former river bed, cobble beach or another environment where the constituent clasts were rounded. Breccia is somewhat similar rock with angular clasts. ^ Coquina is a detrital limestone consisting of shells or shell fragments. Coquina is a coarse-grained (particles over 2 mm in size) conglomeratic rock. Finer carbonate rock made of fossil fragments is calcarenite. ^ Diamictite is a poorly sorted or non-sorted terrigenous non-calcareous sedimentary rock. Diamictite is composed of widely variable clast sizes from clay to boulders. Diamictite is a nongenetic term (purely descriptive, no formation mechanism is implied). Most diamictites are glacial in origin (tillite), but some are volcanic (lithified lahars), submarine (turbidites), mass wasting breccias, etc.

^ Diatomite (diatomaceous earth) is a light-colored sedimentary rock composed chiefly of siliceous shells (frustules) of diatoms. Diatomite is soft and friable rock. Most diatomite deposits are marine but some were formed in lakes. Diatomite has several industrial uses. ^ Dolomite (dolostone) is a carbonate rock containing more than 50% by weight of the mineral dolomite. Dolomite may be a reservoir rock for hydrocarbons (oil & gas). ^ Evaporite is any rock type formed as a result of consolidation of salts crystallized from hypersaline water. Evaporites may form in lagoons where evaporation is intensive due to hot climate and the influx of new lower salinity water is (periodically) restricted, but in many cases evaporites formed by replacing the original minerals of sedimentary rocks with the new ones from brines percolating the rocks. Rock salt and gypsum (as a rock type) are examples of evaporites. ^ Flint is basically a synonym of chert. The term is especially often used for a homogenous dark gray or black variety of chert.

Flint is more widely used name for the rock in general public, but the term “chert” is usually preferred in geological literature (regardless of the color of the rock). ^ Graywacke is an old and vaguely defined rock name for a coarse-grained firmly indurated sandstone that is dark-colored and contains lots of clay and lithic fragments. Graywacke is poorly sorted marine rock. It is nowadays interpreted to be of turbiditic origin. This rock name has been extensively criticized since 1839 when Roderick Murchison said that “it has already been amply shown that this word should cease to be used in geological nomenclature, and…is mineralogically worthless”. Almost two centuries later many geologists seem to share the view of Murchison. But regardless of that, the rock name continues to be used at least informally and as a field term for a dark-grained “dirty” sandstone about which “you can’t tell much about in the field”. ^ Gypsum as a rock type is an aggregate of gypsum crystals. Gypsum is an evaporite and is usually associated with other evaporite minerals like halite and anhydrite.

Gypsum occurs in different forms: massive alabaster, fibrous satin spar, selenite crystals. Gypsum is an important industrial mineral. It is used mostly in the construction industry. ^ Laterite is a reddish weathering product of basaltic (mostly) rocks. Laterite forms in hot to temperate climate and is a residual product of weathering. ^ Lignite is a variety of low-quality coal that is intermediate in coalification between coal and sub-bituminous coal. Brown coal is a synonym of lignite. ^ Limestone is a sedimentary rock that is chiefly composed of calcium carbonate. Limestone is a major rock type. It is mostly marine and biogenic, but can also form as a chemical precipitate (travertine, tufa). Oolite, chalk, calcarenite, coquina, etc. are varieties of limestone. ^ Oil shale is a rock that yields oil (shale oil) upon heating without oxygen. Organic (burning) matter in oli shale is kerogen. Oil shale is a variety or shale. Oil shale reserves are abundant, but it is seldom used as a fossil fuel because it is usually low in calorific value and contains lots of impurities.

^ Oolite is a rock made up of small, generally rounded grains with internal growth bands that are named ooids. Most oolites are limestones (ooids are composed of calcium carbonate). ^ Phosphorite is a sedimentary rock containing significant amount of phosphate material. Phosphorite is a rock containing enough phosphate to be of economic interest. Phosphatic matter may be in ooids, pellets, nodules, shells, bones, etc. ^ Sandstone is a lithified sand. There are many subtypes of sandstone: arkose, orthoquartzite, grit, flagstone, graywacke, and many others. Sandstone is among the most widespread sedimentary rocks. The most abundant constituent mineral in most sandstones is quartz. ^ Shale is a laminated rock consisting of mostly clay minerals. Shale is a lithified mud. Shale and similar mudstones together make up more than half of all the sedimentary rocks. Shale is a source rock of crude oil and natural gas. ^ Siltstone is an intermediate member between sandstone and shale in both grain size and properties. Loess is a variety of wind-deposited siltstone.

Pure siltstone is composed of very fine dusty grains, but it may contain significant amount of sand and clay. ^ Tillite is a poorly sorted glacial sedimentary rock — consolidated glacial till. Tillite in many cases is referred to as diamictite, although the latter is purely descriptive term and may have several formation mechanisms. ^ Travertine is a chemically precipitated limestone. Stalactites and other speleothems in karst caves are made of this rock type. Travertine is also filling calcitic veins. Tufa is a soft and porous chemically precipitated calcitic rock. ^ Turbidite is not a rock type in a strict sense. It is a sedimentary complex consisting of alternating layers of muddy and silty sediments. Turbidite is inferred to have been deposited underwater by a turbidity current. Flysch is a frequently encountered term in older geological literature that means the same thing. ^ Umber is a marine sediment associated with black smokers. Umber is brown in color (brown earth, darker than ocher) and valued as a natural brown pigment. Umber occurs in Cyprus-type massive sulfide deposits.

^ Metamorphic rocks Metamorphic rocks are derived from other pre-existing rocks by mineralogical and/or structural changes. Metamorphism takes place in the solid state and at elevated pressure and temperature generally at depth in the crust 1 . Amphibolite is a metamorphic rock composed of amphiboles (usually hornblende) and plagioclase. Amphibolite should not be confused with the metamorphic facies of the same name. Amphibolite is a widespread rock type. ^ Anthracite is a type of coal with a highest rank. Coal rank measures the carbon content which is in correlation with calorific value and metamorphic grade. Anthracite has a very high carbon (over 90%) and low volatile content (below 5%). ^ Blueschist is a bluish metamorphic rock containing amphibole glaucophane. Blueschist forms at high pressure and low temperature (subducting slab of lithosphere). Its protoliths are basaltic rocks. ^ Charnockite is an orthopyroxene-bearing anhydrous metamorphic rock. Charnockite is often referred to as granite.

This practice is in most cases misleading because charnockitic rocks did not crystallize from magma. ^ Eclogite is a beautiful rock consisting of bright green pyroxene omphacite and red garnet. Eclogite forms at high-pressure conditions. Its protolith is a basaltic rock. ^ Epidosite is a hydrothermally altered basaltic rock consisting of epidote and quartz. Epidosite is a product of hydrothermal metamorphism (alteration). Its parent rock was a mafic igneous rock (basalt, diabase). ^ Fenite is a metasomatic rock surrounding complexes of carbonatite-alkaline rocks. Fenite is an altered former granitic (or similar) rock. Fenitization is a process which replaces some or all of the original minerals by new and generally more alkaline phases (alkali pyroxene, alkali amphibole, feldspathoids, etc.). ^ Gneiss is a banded coarse-grained metamorphic rock that formed deep in the crust beneath forming mountain ranges. The chemical composition may be highly variable, although most gneissose rocks are granitic. ^ Granulite is a confusing term in metamorphic petrology.

Usually it is a metamorphic rock of granulite facies. This is unfortunate use because metamorphic rocks should never be confused with facies names. There is a similar case with amphibolite and amphibolite facies, but amphibolite is a well-defined rock type. We know that amphibolite does not necessarily equilibrate at amphibolite facies conditions and neither are all amphibolite facies rocks amphibolites. The situation, unfortunately, is more complicated with granulites because there are no compositional defining driteria. Therefore, rocks with widely varying compositions are named so which complicates the matter to the point of utter confusion. It has been proposed that the term should be abandoned and granofels used instead which is free of unwanted associations with metamorphic facies. Nevertheless, the term “granulite” continues to be used in the scientific literature. ^ Greenschist is a schistose metamorphic rock. Green color is given to this rock usually by chlorite, epidote, actinolite, etc. Greenschist is usually metamorphosed mafic igneous rock (metabasite). If the green mineral is chlorite, the rock should be named chlorite schist.

^ Greisen is a pneumatolytically altered granitic rock. Greisen contains lots of quartz and mica plus several exotic minerals like cassiterite, tourmaline, topaz, etc. Greised is a metamorphic rock that metamorphosed itself — hot fluids that altered the rock were provided by the cooling magma body itself. ^ Hornfels is a fine-grained compact rock rock formed by contact metamorphism (low pressure, high temperature). ^ Marble is a metamorphosed (recrystallized) carbonate rock (limestone, dolomite rock). Marble is mostly composed of calcium carbonate. Marble in geology has somewhat more restrictive meaning than in commerce. ^ Metapelite is a metamorphosed claystone (mudstone, shale). Usually either schist or slate is named so although any metamorphosed rock can be metapelite if it has a clayey protolith. These rocks are rich in aluminum and contain various Al-rich silicate phases like micas, garnet, staurolite, kyanite, cordierite, etc. ^ Migmatite is an inhomogenous rock consisting of dark-colored metamorphic rock and light-colored igneous leucosomes.

Igneous or igneous-looking component of the rock is either the result or partial melting or material intruded into the rock. ^ Mylonite is a highly deformed rock that is composed of stretched and flattened mineral grains. Mylonites form deep in the crust where temperature is high enough for ductile deformations in shear zones. ^ Phyllite is a metamorphic rock (usually metapelite) intermediate in grade between slate and schist. Minute platy crystals of sericite, chlorite, or graphite give a characteristic sheen to the cleavage surfaces which distinguishes it from duller slate. Constituent minerals in schist are coarser. ^ Quartzite is a metamorphic rock consisting largely or entirely of quartz 2 . In vast majority of cases, quartzite is a metamorphosed sandstone. ^ Schist is a strongly foliated rock rich in platy or elongated minerals. Schist forms during regional metamorphism. Composition may be highly variable. ^ Serpentinite is a metamorphosed ultramafic rock. It is composed of serpentine minerals.

Serpentinite forms at the expense of olivine- and pyroxene-bearing rocks (peridotite, pyroxenite, dunite). Chlorite, talc and magnesite frequently occur in these rocks. ^ Shungite is a metamorphosed oil shale consisting of structureless carbon. Shungite is a very rare rock type and the details of its genesis are unclear. ^ Skarn is a metasomatic rock composed of calc-silicate minerals. Skarn is formed at the contact zone between carbonate rocks and silicic magma. Skarn is often rich in hydrothermal ore minerals. ^ Slate is a fine-grained rock that can be split into slabs (slaty cleavage). Slate in the majority of cases is a compacted and metamorphosed mudstone or shale. Slate is a low- to medium-grade metamorphic rock that will pass on to phyllite, schist and gneiss with increasing burial depth. ^ Soapstone is a soft rock composed of talc with other silicates like micas, pyroxene, amphibole, chlorite, etc. Soapstone is named so because of slippery feel when touched.

Soapstone may be schistose (talc schist) or massive. ^ Suevite is a shock-metamorphosed rock. Suevite is an impact breccia composed of impact melt and lithic fragments. Impact metamorphism is caused by a collision between Earth and large extraterrestrial body which is usually meteorite or asteroid. ^ References 1. Jackson, J. A. (1997). Glossary of Geology , 4th Edition. American Geological Institute. 2. Barth, T. F. W. (2007). Quartzite. In: McGraw Hill Encyclopedia of Science & Technology , 10th Edition. McGraw-Hill. Volume 14. 722. 3. Tilling, Robert I. (2007). Ignimbrite. In: McGraw Hill Encyclopedia of Science & Technology , 10th Edition. McGraw-Hill. Volume 9. 20-21. Granite - Yosemite National Park (U.S. National Park Service) Granite Granite Granite Granite dominates the geology of Yosemite National Park.

The park boundary perfectly frames a landscape that is composed almost entirely of granite, and it’s no surprise that this landscape contains some of the most iconic rock formations on earth. The bedrock geology of Yosemite National Park consists almost entirely of granitic rocks. Granite is an intrusive igneous rock, which means it crystallized from molten rock, called magma, miles underground. At these depths, magma is insulated by the rocks around it and cools very slowly, growing large interlocking crystals. Granite is often said to have a "salt-and-pepper" appearance: the lighter-colored minerals are quartz, potassium feldspar, and plagioclase feldspar, and the darker-colored minerals are mostly biotite and hornblende. There are also minor amounts of various accessory minerals such as titanite and zircon. The major minerals that are present in granite as displayed in a sample of Half Dome Granodiorite. Looking at a granitic rock's composition tells geologists much about its formation. Different granites have unique chemical compositions that produce different mineral proportions. In fact, granitic rocks are generally classified by their mineral proportions and given names such as granite, granodiorite, tonalite, and diorite.

A clear example of contrasting compositions is visible on the southeast face of El Capitan where the dark-colored hornblende-rich Diorite of North America intruded the older and lighter-colored quartz and feldspar-rich El Capitan and Taft granites. Granitic rocks of different compositions (and colors) are juxtaposed on the southeast face of El Capitan. The darker rock in the center of the photograph is the Diorite of North America. The thermal history of a granite—how fast it cooled and how long it spent at different temperatures—dictates the size of the mineral grains, and thus the rock texture. A prime example is the Cathedral Peak Granodiorite, which has unusually large (up to 3 inches long) crystals of potassium feldspar that are thought to have formed through episodic heating and cooling of the magma. This type of rock dominates the landscape in the Yosemite high country near Tuolumne Meadows. Formation Granite is most often formed where oceanic plates dive beneath continental plates in tectonic environments called subduction zones. In these zones, the input of water-rich oceanic crust causes partial melting at the base of the crust about 25 to 30 miles below the earth’s surface.

The resulting magma is less dense than material typically found at that depth, so it rises like oil in salad dressing. This is the silica-rich magma that is often violently expelled from the large volcanos, like Mt. Saint Helens and Mt. Rainier, which sit above subduction zones today. Diagram of subduction beneath the ancestral Sierra Nevada about 100 million years ago. Illustration by Eric Knight. Plutons Approximately three to 10 miles below these volcanoes are the magma chambers that feed them, which build up over time as bodies called plutons. This rock, once it cools, will become the type of granite that is exposed in Yosemite. Plutons are large (often covering over 400 square miles) and are typically formed by many pulses of magma over a few million years. The granite that composes an individual pluton will have a relatively homogeneous texture and composition, displaying minor variations across its extent. There are at roughly 35 individually defined plutons in the Yosemite area. Plutons are usually named after a famous landmark that is dominantly made of that rock (examples include the Half Dome Granodiorite and the El Capitan Granite).

Intrusive Suites and Batholiths A group of plutons that are considered to be part of the same magmatic event and thus related are called intrusive suites. Intrusive suites typically are emplaced over about 10 million years and are often interpreted as being the roots of a single volcanic system. There are seven intrusive suites in Yosemite National Park. The Tuolumne Intrusive Suite is the youngest and most extensive, occupying about 1/3 of the park’s area. The Tuolumne Intrusive Suite contains many famous rock types including the Half Dome Granodiorite and Cathedral Peak Granite. Geologic map of Yosemite Valley showing the extents of different granitic rocks and their respective intrusive suites. A large amalgamation of plutons is called a batholith. Yosemite is situated within the 70-mile-wide, 300-mile-long Sierra Nevada Batholith. Most of the granite in the Sierra Nevada Batholith was emplaced between 120 and 85 million years ago during the Jurassic and Cretaceous periods. This is one of the most rapid periods of continental crust assembly known.

In fact, it has been speculated that this voluminous volcanism may have contributed to the mass extinction during the middle of the Cretaceous period during which most large marine dinosaurs (such as ichthyosaurs and plesiosaurs) perished. Hot Research Few places on earth display the internal complexity of a batholith better than El Capitan. Even though Yosemite Valley has received considerable attention because of its geologic fame and dominant place in our cultural geography, the 3,000-foot-tall southeast face of El Capitan remained unmapped until recently. Between 2011 and 2013, a team of geologists completed the first geologic map of this sheer face. The map was made using a combination of techniques including remote sensing, climbing, and geochemistry. The map reveals many details about the vertical structures of plutons and the complex interactions between intrusive suites. Geologic map of the southeast face of El Capitan showing the complexity of the granitic rocks there. This complexity reveals intricacies of magma intrusion that occurred miles below the surface millions of years ago. A geologist maps granitic rocks 2,000 feet above the floor of Yosemite Valley on El Capitan. Photo by Chris Gibbisch.

Many national parks were founded for their geology, and Yosemite is known throughout the world for its exceptional high cliffs and rounded domes. Visitors to the park, from hikers to rock climbers, experience a landscape dominated by granite. Duration: Igneous Rocks | Pictures of Intrusive and Extrusive Rock Types Pictures of Igneous Rocks Photos of Common Intrusive and Extrusive Igneous Rock Types Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende , pyroxene, and biotite . The specimen shown is about two inches (five centimeters) across. What are Igneous Rocks? Igneous rocks are formed from the solidification of molten rock material. There are two basic types. Intrusive igneous rocks crystallize below Earth's surface, and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks are diorite , gabbro , granite , pegmatite , and peridotite . Extrusive igneous rocks erupt onto the surface, where they cool quickly to form small crystals. Some cool so quickly that they form an amorphous glass.

These rocks include andesite , basalt , obsidian , pumice , rhyolite , scoria , and tuff . Pictures and brief descriptions of some common igneous rock types are shown on this page. Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende , and sometimes quartz . The specimen shown above is about two inches (five centimeters) across. Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene. The specimen shown is about two inches (five centimeters) across. Gabbro is a coarse-grained, dark-colored, intrusive igneous rock that contains feldspar, pyroxene, and sometimes olivine . The specimen shown above is about two inches (five centimeters) across. Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form. The specimen shown above is about two inches (five centimeters) across.

Granite is a coarse-grained, light-colored, intrusive igneous rock that contains mainly quartz , feldspar, and mica minerals. The specimen above is about two inches (five centimeters) across. Peridotite is a coarse-grained intrusive igneous rock that is composed almost entirely of olivine . It may contain small amounts of amphibole, feldspar, quartz , or pyroxene. The specimen shown above is about two inches (five centimeters) across. Pegmatite is a light-colored, extremely coarse-grained intrusive igneous rock. It forms near the margins of a magma chamber during the final phases of magma chamber crystallization. It often contains rare minerals that are not found in other parts of the magma chamber. The specimen shown above is about two inches (five centimeters) across. Rhyolite is a light-colored, fine-grained, extrusive igneous rock that typically contains quartz and feldspar minerals. The specimen shown above is about two inches (five centimeters) across. Pumice is a light-colored vesicular igneous rock. It forms through very rapid solidification of a melt. The vesicular texture is a result of gas trapped in the melt at the time of solidification.

The specimen shown above is about two inches (five centimeters) across. Scoria is a dark-colored, vesicular, extrusive igneous rock. The vesicles are a result of trapped gas within the melt at the time of solidification. It often forms as a frothy crust on the top of a lava flow or as material ejected from a volcanic vent and solidifying while airborne. The specimen shown above is about two inches (five centimeters) across. Fire Opal is sometimes found filling cavities in rhyolite . Long after the rhyolite has cooled, silica-rich ground water moves through the rock, sometimes depositing gems like opal , red beryl , topaz , jasper, or agate in the cavities of the rock. This is one of many excellent geological photographs generously shared through a Creative Commons License by Didier Descouens. The best way to learn about rocks is to have a collection of specimens to examine while you study. Seeing and handling the rocks will help you understand their composition and texture much better than reading about them on a website or in a book. The Geology.com store offers inexpensive rock collections that can be mailed anywhere in the United States or U.S. Territories.

Mineral collections and instructive books are also available. Welded Tuff is a rock that is composed of materials that were ejected from a volcano, fell to Earth, and then lithified into a rock. It is usually composed mainly of volcanic ash and sometimes contains larger size particles such as cinders. The specimen shown above is about two inches (five centimeters) across. More Rocks The 3 basic rock types Ask GeoMan... What are the 3 basic types of rocks? Just as any person can be put into one of two main categories of human being, all rocks can be put into one of three fundamentally different types of rocks. They are as follows: Igneous Rocks Igneous rocks are crystalline solids which form directly from the cooling of magma. This is an exothermic process (it loses heat) and involves a phase change from the liquid to the solid state. The earth is made of igneous rock - at least at the surface where our planet is exposed to the coldness of space. Igneous rocks are given names based upon two things: composition (what they are made of) and texture (how big the crystals are). Click here for more on igneous rock composition and texture.

Click here for more on elements and minerals common in igneous rocks. Click here for more on magma and igneous rocks. Click here for more on plate tectonics and the formation of magma. Click here for a chart summarizing the main divisions of igneous rocks. Click here for more on basalt and granite. Sedimentary Rocks In most places on the surface, the igneous rocks which make up the majority of the crust are covered by a thin veneer of loose sediment, and the rock which is made as layers of this debris get compacted and cemented together. Sedimentary rocks are called secondary, because they are often the result of the accumulation of small pieces broken off of pre-existing rocks. There are three main types of sedimentary rocks: Clastic: your basic sedimentary rock. Clastic sedimentary rocks are accumulations of clasts: little pieces of broken up rock which have piled up and been "lithified" by compaction and cementation. Chemical: many of these form when standing water evaporates, leaving dissolved minerals behind. These are very common in arid lands, where seasonal "playa lakes" occur in closed depressions.

Thick deposits of salt and gypsum can form due to repeated flooding and evaporation over long periods of time . Organic: any accumulation of sedimentary debris caused by organic processes. Many animals use calcium for shells, bones, and teeth. These bits of calcium can pile up on the seafloor and accumulate into a thick enough layer to form an "organic" sedimentary rock. Click here for more on sedimentary processes and rocks (RCC). Click here for more on sedimentary rocks (GPHS). Metamorphic Rocks The metamorphics get their name from "meta" (change) and "morph" (form). Any rock can become a metamorphic rock. All that is required is for the rock to be moved into an environment in which the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions. In most cases, this involves burial which leads to a rise in temperature and pressure. The metamorphic changes in the minerals always move in a direction designed to restore equilibrium. Common metamorphic rocks include slate, schist, gneiss, and marble. Click here for more on metamorphic processes and rocks (RCC). Click here for more on metamorphic rocks (GPHS).

Question: What type of rock is granite?
Answer: Igneous